This varying degree to which shared genetic factors influence cannabis and tobacco use and misuse is partly dependent on the stage of cannabis and tobacco involvement that is under study – as shown in Figure 3. Genetic factors reflecting a predisposition to disinhibition, impulsivity, sensation-seeking and general problem behavior are more likely to contribute to early onset of cannabis and tobacco use (37;38). Maintenance of substance use followed by development of problems (e.g. cannabis abuse/dependence and nicotine dependence) is also genetically influenced, but these genes may be less related to disinhibition, and more so, to positive reinforcement and shared features of abuse/dependence (e.g. common genetic influences on cannabis and tobacco withdrawal symptoms). Two influential studies examined the impact of these potentially etiologically distinct sources of genetic overlap by examining the shared genetic influences across cannabis use, nicotine use, cannabis abuse/dependence and nicotine dependence. Importantly, these studies specified that a proportion of the genetic influences on nicotine dependence was in common with and contingent on exposure to nicotine use (39). When such a contingency was imposed on both cannabis and tobacco
